jumprun | Run scripts from anywhere in terminal

This is my first question, the reason i signed up to the site. I'm developing a game using Qt 5.9 and I use QTimer to spawn enemies on the screen. Everytime the timer's timeout function is called, an enemy is spawned. What i try to do is if a player kills let's say 10 enemies, the timer interval decreases, so the enemies will spawn more frequently, making the game a little bit more challenging. The first time the timer interval is set, the game runs perfectly, but the second time the setInterval() method is called, when the player kills 10 enemies, the game suddenly crashes. I tried debugging it to figure out what might cause it, and it seems that it crashes when i try to set the spawnInterval. I'm fairly new to coding so any advice is appreciated! Here are the relevant source files and codes from my code:

main.cpp